Polyethylene terephthalate (PET) resin is widely utilized in manufacturing industrial yarn. Industrial yarn made utilizing PET usually has much higher modulus and tenacity than textile yarn made utilizing PET. Industrial yarn usually also has a much higher denier than textile yarn. For example, industrial PET yarns commonly possess a tenacity of at least 6.2 cN/dtex (centinewtons/decitex) and have a dtex of about 833 to about 2220, while textile polyester yarns commonly have a tenacity of only about 3.0 to 4.0 cN/dtex and have a decitex of about 111 to about 556. It is important for industrial yarns to have higher levels of modulus and tenacity to be useful as reinforcements for manufactured articles, such as tires, hoses, belts, and the like. Such industrial yarns are of particular value as reinforcements for tires, conveyor belts, and power transmission belts.
In many applications it is also important for industrial yarns to exhibit dimensional stability as well as high modulus and high tenacity. It has been widely recognized that higher melt spinning speeds usually result in the production of yarns which exhibit lower shrinkage. Unfortunately, the utilization of increased melt spinning speeds results in yarns which have reduced tenacity. Uncreased melt spinning speeds have accordingly not proven to be an acceptable means for commercially producing industrial yarns which exhibit low levels of shrinkage in combination with high tenacity. In fact, heretofore, melt spun filaments have been formed through the utilization of relatively low stress spinning conditions to yield spun filaments having relatively low birefringence of less than about 0.03. Such melt spun filaments are particularly amenable to subsequent hot drawing procedures whereby the required tenacity values are ultimately developed. Such as-spun filaments are commonly subjected to subsequent hot drawing which may or may not be conducted in-line when forming textile as well as industrial fibers to develop the desired tensile properties. Drawing procedures which are carried out subsequent to the melt spinning process can have a significant effect on drawn yarn shrinkage. However, drawing procedures alone cannot typically be used to significantly improve yarn dimensional stability.